- Per-user SSE cap uses per-connection leases in a sorted set instead of
a shared INCR/DECR counter with a TTL. A stream that outlived the TTL
could let the counter expire, then decrement another stream's slot or
drive the count negative. Leases refresh while a stream sends frames
and age out when a stream dies without cleanup.
- Shielded cleanup is bounded per step: stream close and on_close in
ClosingStreamingResponse, unsubscribe and close in AsyncTopic, lease
release, and the replay-budget check, so a dead Redis connection can't
hold a request or a graceful shutdown.
- Artifact downloads send an ASCII filename plus an RFC 5987 filename*
for non-ASCII names, build headers before opening the file, and close
disk handles off the event loop.
- Type hints and docstrings on the new helpers.
